The present invention pertains to pumps of the piston type and for use, for example, in apparatus shown in my U.S. Pat. No. 4,015,618 of Apr. 5, 1977, or for example to supply a chlorine solution for swimming pools, as well as for many other uses. For example, small pumps have been used for cleaned-in-place dairy equipment and are referred to in U.S. Pat. No. 4,047,851 of Sept. 13, 1977 which has been assigned to an assignee common with said 4,015,618 patent and with the present application. One shortcoming of the bellows type pump shown in the '851 patent or the pump shown in the '618 patent was that it was impossible for those pumps to discharge into a pressure vessel or pressure line, but rather those pumps could only pump to an atmospheric container. Another short-coming of prior art pumps in general was that they did not maintain their prime particularly when the vessel from which they were drawing their supply fluid became empty. As a result it was often necessary to disassemble the pump and its lines, bleed air from the system, and then again prime the pump. Still a further disadvantage of prior art pumps was that they were not adjustable as to their output, particularly when they were required to pump either to a pressurized outlet or to atmosphere.